Technical Requirements for MBR Membranes Explained

Against the backdrop of growing water scarcity and increasingly stringent environmental regulations, Membrane Bioreactor (MBR) technology has become a leading choice for municipal sewage and industrial wastewater treatment due to its excellent effluent quality, small footprint, and low excess sludge production. However, to fully leverage the advantages of MBR membranes and ensure long-term, stable, and efficient system operation, a series of key technical requirements must be strictly followed. This article details the technical essentials for MBR membrane applications, covering membrane material selection, operational parameter control, pretreatment processes, and maintenance management.

1. Membrane Material and Module Selection

The core of an MBR system is the membrane module; its material and structure directly determine treatment performance and service life.

First, the membrane material must possess good mechanical strength, chemical stability, and anti-fouling properties. Currently, polyvinylidene fluoride is the dominant membrane material due to its excellent chemical resistance, oxidation resistance, and mechanical strength, allowing it to withstand frequent chemical cleaning and aeration scouring. In addition, the membrane pore size is typically required to be between 0.01 and 0.4 micrometers, falling within the microfiltration or ultrafiltration range, effectively retaining suspended solids, bacteria, and some viruses.

Second, the structural configuration of the membrane module is critical. Common MBR membrane modules include hollow fiber membranes and flat sheet membranes. Hollow fiber membranes offer high packing density and a small footprint, making them suitable for large-scale projects, but they require stricter pretreatment and are prone to sludge accumulation and fiber breakage. Flat sheet membranes, in contrast, exhibit stronger anti-fouling capability, easier maintenance, and longer service life, though they come with a relatively higher initial investment. Users should select the appropriate membrane module type based on water quality characteristics, project scale, and operational capacity.

2. Strict Pretreatment Requirements

Pretreatment is the first line of defense for stable MBR membrane operation. Raw water entering the MBR system must undergo rigorous pretreatment to remove substances that could cause mechanical damage or chemical degradation to the membranes.

Specifically, the raw water should pass through screens to remove large particles; the screen gap is typically required to be less than 1–2 millimeters. For wastewater containing significant amounts of fibers or hair, an ultra-fine screen or sieve should be installed. Additionally, the oil and grease content entering the bioreactor must be strictly controlled, as grease can easily form a film on the membrane surface, leading to irreversible fouling. It is also essential to ensure that the raw water does not contain organic solvents, strong oxidants, or other chemicals that could damage the membrane material. For industrial wastewater, facilities such as equalization tanks and primary sedimentation tanks are often needed to homogenize water quality and reduce the suspended solids load.

3. Precise Control of Operational Parameters

Stable operation of an MBR system relies on precise control of key parameters.

- Mixed Liquor Suspended Solids and Sludge Retention Time: MBR systems typically operate at high mixed liquor suspended solids concentrations, generally controlled between 8,000 and 15,000 mg/L, or even higher. However, excessively high MLSS increases the risk of membrane fouling, so it must be reasonably managed based on actual operating conditions. Meanwhile, the sludge retention time is usually long, often 30–60 days, which helps reduce excess sludge production, but care must be taken to avoid deterioration of mixed liquor properties due to sludge aging.

- Aeration Intensity: Aeration in an MBR system serves a dual purpose: it provides oxygen for microorganisms to perform biodegradation and creates turbulence through bubble scouring to inhibit foulant deposition on the membrane surface. Typically, membrane scouring aeration is a key parameter for controlling membrane fouling and should be set based on membrane area and operating flux. Insufficient aeration cannot effectively clean the membrane surface, while excessive aeration wastes energy and may damage the membrane fibers.

- Operating Flux and Transmembrane Pressure: Operating flux refers to the volume of permeate produced per unit membrane area per unit time; its selection directly affects the rate of membrane fouling. During design, the critical flux should be determined based on water quality characteristics and temperature. Typically, the actual operating flux should be maintained below the critical flux. Transmembrane pressure is a direct indicator of the degree of membrane fouling; when TMP rises to a set threshold, chemical cleaning is required.

- Temperature and pH: The biological treatment process is significantly affected by temperature, especially in winter when low temperatures reduce microbial activity, impacting treatment efficiency and membrane flux. The pH should be maintained within a range suitable for microbial growth, generally 6–9, while also meeting the tolerance limits of the membrane material.

4. Scientific Cleaning and Maintenance Regimen

Membrane fouling is inevitable during MBR system operation, so a scientific cleaning and maintenance regimen must be established.

Maintenance cleaning is typically performed 1–2 times per week by adding low concentrations of chemical agents such as sodium hypochlorite to the membrane modules to remove organic foulants and microorganisms from the membrane surface in situ. Recovery cleaning is carried out every 3–6 months; when TMP significantly increases or permeate production noticeably declines, the membrane modules are either taken out or cleaned offline using higher concentrations of acids, bases, oxidants, etc., to thoroughly remove stubborn foulants from inside and outside the membranes.

In addition, routine inspections are essential. Regular checks should be conducted to verify the integrity of membrane modules, monitor effluent quality, observe aeration uniformity, and promptly identify and address issues such as broken fibers, clogged aerators, and pipeline leaks.

5. Conclusion

As an efficient water treatment technology, MBR holds broad application prospects. However, to make it both effective and durable, strict adherence to technical requirements is essential at every stage—from design and construction to operation. Only by achieving scientific and standardized practices in membrane material selection, pretreatment processes, operational parameter control, and cleaning and maintenance can the full advantages of MBR membranes be realized. This ensures long-term stable effluent quality and optimal investment returns, providing solid technical support for water resource recycling and ecological environmental protection.